This invention relates to the continuous molten metal casting arts, and more particularly to wheel-band casting machines wherein the mold geometry is substantially square, triangular, rectangular, or trapezoidal. Certain embodiments of this invention may also be used with conventional twin belt or/and plate molds or other vertical casting machines which normally a mold cavity containing corners and thus form a cast bar having sharp edges.
It is well known in the prior art that combinations of the following factors are closely interrelated to the presence or absence of bar cracks, especially corner cracks, in cast steel bar produced by typical metal casting machines: the composition of the metal or alloy being cast, especially alloys in which precipitation of some constituents is a problem; mold geometry and dimensions; speed of casting; overall cooling rate and uniformity of bar cooling. Of these factors, only the metal composition being cast is fixed by preference; except that the mold geometry cannot usually be changed during operation and then only with extreme difficulty. Many other parameters are variable. With conventional wheel-band casting machines the trapezoidal shaped mold cross section with radiused bottom corners has long been recognized as the optimum shape for wheel life, transverse solidification pattern, and bar extraction from the mold (see for example U.S. Pat. No. 3,818,972 and 3,834,444 by Milton Berry and assigned to the assignee of the present invention). However, trapezoidal or square mold cross sections undergoing even slight bending while hot may tend to produce longitudinal or transverse bar cracks with certain metal alloys such as steel, even when cooling rates, or other combinations of parameters are varied over their permissable ranges.
These cracks are attributed to the difficulty of obtaining certain combinations of the above variable casting characteristics, especially heat transfer, casting rate, alloy constituents with a fixed mold geometry. It is known from prior art that some factors are more important than others. For example, when certain constituent elements are precipitated to the grain boundaries they then solidify at a cooler temperature than the predominant constituents. Due to the heat transfer characteristics of conventional square or trapezoidal molds, the intermetallic boundary forms along a longitudinal line extending from the corners, especially the sharp corners of the trapezoidal mold, thereby increasing the risk of bar cracking at this point. Various references in the prior art suggest changing the alloy composition to change the precipitation patterns or changing other factors such as cooling or casting rates to avoid such cracking. This invention is directed at alternate means to reduce or eliminate cracking, thus avoiding the complexities of chemical adjustment and eliminating the uncertainty of changing cooling characteristics for each of several alloys. It also permits continuous casting of otherwise difficult alloy combinations.
Further, in some prior art processes, mechanical corner preparation, such as scalping, is required to facilitate optimum rolling of the cast bar stock into rod even if all parameters have been adjusted to minimize corner cracking due to other corner defects such as excessive porosity or excess flash or fins (i.e., material which has leaked between the wheel and band and solidified to form a thin metal fin) as illustrated in U.S. Pat. Nos. 3,780,552 and 3,469,620.
U.S. Pat. No. 3,736,977 dicloses the use of combustible and non-combustible tapes or the like introduced into the mold corners of plate molds used for vertical casting. However, in the use disclosed in this patent, the tapes are used solely to prevent leakage of molten metal through the gaps formed at the butt joints of the plate mold due to differential thermal expansion of the plates. Molten metal spills and breakthroughs can occur in such a casting method if no such precautions are taken. The instant invention, however, is directed to an entirely different use for the introduction of supplemental material in the mold cavity.